Malignant melanoma is one of the fastest increasing cancers in the United States and no curative treatment is yet available. Solar ultraviolet (UV) radiation, especially childhood sun exposure is an important etiological risk factor of melanoma. Retinoid X Receptor a (RXRa), a member of the nuclear receptor (NR) superfamily, is a central coordinator for transducing diverse cellular signals. In the context of studying the role of RXRa in skin, we have discovered an unexpected and novel role for this NR in melanomagenesis: RXRa[ep-/-] mice, specifically lacking RXRa in epidermal keratinocytes, develop melanocytic growths (MGs) resembling melanoma at high frequency when subjected to a two-step chemical carcinogenesis protocol (DMBA+TPA). Our results suggest that RXRa may regulate a keratinocyte ? melanocyte signaling pathway(s) implicated in the control of melanocytic proliferation. Thus, we have generated a new mouse model for melanomagenesis. However, the molecular mechanisms that underlie these activities of RXR are not known. Given the importance of keratinocytes in regulating melanocyte mitogenesis, understanding how this regulation becomes aberrant in melanoma is significant, since it can possibly lead to the development of effective therapeutic strategies to counteract melanoma formation and progression. Our long-term goal is to identify the mechanisms of signal transduction between keratinocytes and melanocytes that contribute to the development of melanoma. Based on the above observations and from the preliminary data, we propose the following two specific aims. First, we propose to elucidate the cellular and molecular mechanisms by which keratinocytes control melanocyte mitogenesis and transformation leading to a malignant phenotype. Our working hypothesis is that RXRa, directly or indirectly, represses keratinocytic expression of endothelin 1 (ET-1), SCF, POMC and FGF2 which may serve to regulate melanocytic mitogenesis in a paracrine manner. Second, we propose to identify intracellular targets (melanocytic factors) that control melanocyte homeostasis and UV-induced melanomagenesis. Our working hypothesis is that melanocytic factors, such as cyclin dependent kinase-4 (Cdk4), may modulate the responsiveness of these cells to the mitogenic effects of keratinocyte-derived paracrine factors. We believe that our efforts in the context of the work described herein will lead to a detailed understanding of the mechanism(s) by which melanocyte mitogenesis and melanomagenesis are regulated by keratinocytic RXRa, and perhaps other paracrine factors. The proposed project is potentially innovative as our laboratory generated the RXRa [ep-/-] mouse that has been used for these studies, and was the first to characterize the in vivo role of RXRa in skin during epidermal homeostasis. This contribution is significant and the results are expected to have a positive impact on human health, because the outcome of the work will provide the molecular cornerstone for the development of future pharmacological strategies designed to treat, and ultimately cure malignant melanoma.